The standard method of growing fish in inland water bodies is in open ponds, either free range or in cages. Such ponds take advantage of the land resources particularly in rural areas, however, they may have disadvantages. Free-range fish grown in such ponds are difficult to observe, monitor and manage. For example, it is difficult to assess the feed intake of the fish, their health and growth rate. Harvesting fish from such a wide area is also difficult as either the pond needs to be drained (a process which may take many hours) or a large net dragged through the water. Predation by birds and water rats is also highly problematic. Arguably the greatest disadvantage is pollution of the water through nutrients from uneaten food and fish waste. The pollution typically limits the pond yields to 1-2 tonnes per hectare—particularly when water exchange rate through the pond is low.
Nutrients limit a pond's yield by encouraging blooms of microalgae. As nutrient input into the pond increases, so does the strength of the blooms. The bloom strips oxygen from the water at night time leading to many fish kills. If the bloom of microalgae gets too thick, the algae will die, sink to the bottom of the pond and again strip oxygen from the water. Solid waste excreted by the fish and uneaten food creates a sludge on the bottom of the pond that also depletes valuable oxygen. This sludge further complicates the net harvesting process because as the net is dragged through the pond it stirs up the sludge, further depleting oxygen and killing fish.
In an effort to overcome some of the disadvantages of open pond culture, people often contain fish within net cages. That overcomes the problems of harvesting, predation and fish management. Nevertheless yields are still limited by nutrient input into the pond and in the case of bloom induced oxygen depletion, fish are actually worse off in cages, as the fish are crowded together and oxygen starvation therefore occurs rapidly. The density of the fish in these cages is limited to approximately 10 kg/m3.